The present invention relates to a control device for effecting variable hydraulic fluid delivery to a gearshift element of an automatic transmission in an automobile, for controlling the actuation of the gearshift element.
Automatic transmissions are known which have a gearshift element, e.g. a so-called forward gear clutch, which connects positively the hydrodynamic torque converter turbine and the drive gears, when one of the forward gears is connected, and which is disconnected during certain vehicle operating conditions. By way of example, a hydraulic forward clutch element may be disengaged when the vehicle is coasting, by releasing the accelerator pedal, so as to disconnect the engine from the wheels. The engine can then idle (or may be stopped) during such operating states to conserve fuel. When it is desired to again drive the wheels with the engine, the vehicle operator depresses the accelerator pedal, and pressurized hydraulic fluid is provided to the gearshift element to re-connect the engine to the transmission and thereby to the driving wheels.
DE-OS No. 25 02 347 discloses a device for controlling the rate of delivery of hydraulic fluid to a gearshift element of an automatic transmission for actuating the same, in which the rate of delivery is controlled as a function of the pressure upstream from the gearshift element. The purpose of such a device is twofold: such a device permits engagement of the gearshift element as rapidly as possible and at the same time, engagement occurs with sufficient ease in order to avoid sudden shifting jolts, namely, those that would result from suddenly connecting an idling (or a stopped) engine to the rotating wheels of a moving vehicle. Such a control device includes a fluid delivery line with a throttle, a bypass line for bypassing the throttle, and a valve located in the bypass line for opening and closing the bypass line.
In operation, when pressurized fluid is initially supplied to the fluid delivery line, to re-connect the gearshift element, the fluid pressure upstream of the bypass valve is relatively high, and pressure downstream of the valve, i.e., at the gearshift element, is relatively low. This causes the bypass valve to open, so that initially pressurized fluid is delivered to the gearshift element through both the throttle line and the unrestricted bypass line, and pressure at the gearshift element builds up rapidly. The piston of the gearshift element moves very rapidly to a point where the element begins to engage because the bypass line is open. However, the increased fluid pressure on the downstream side of the bypass valve, which has caused the element to start to engage, also causes the bypass line to close before full clutch engagement occurs. Thereafter, pressurized fluid can flow to the gearshift element only through the throttle. Further pressure buildup, to complete the engagement of the element, occurs more slowly. A smooth engagement of the gearshift element is achieved due to the two stage engagement process, i.e., a rapid initial pressure build-up followed by a slower final pressure build-up in the element.
Control of the hydraulic fluid delivery solely by the pressure upstream from the gearshift element is insufficient to prevent sudden shifting jolts if the gearshift element is actuated while the vehicle is moving. Such actuation is required, for example, when operating the vehicle in the manner described above in order to obtain fuel savings during certain vehicle operating states (during coasting or engine-braking operations) and the gearshift element is disengaged while the vehicle is moving. After the particular vehicle operating state is terminated, it is necessary to re-engage the gearshift element, e.g. the forward clutch of the automatic transmission, in order to re-connect the driving engine and the driven wheels. However, as discussed above when the gearshift element is disengaged, the engine is normally idling. It is necessary then to connect an idling engine to moving engine wheels.
A sudden re-engagement of the gearshift element would effect a jump in engine rpm, since the engine and the wheels would be forced to turn at the same rate, taking into account the transmission ratio. Such re-engagement and engine acceleration can occur without any noticeable jolt at vehicle speeds up to about 60 to 80 kilometers per hour. However, at higher speeds, such re-engagement will cause the vehicle to lurch and produce an uncomfortable ride.